It is generally desired in association with the production of cellulose pulp from chips to first pre-treat the chips with steam such that air can be expelled. If this is carried out in a satisfactory manner, a homogenous impregnation of the chips is facilitated, and this gives a better and more even quality of pulp and a lower reject quantity. It is also possible to achieve a better transit of the column of chips through a continuous digester if all air has been expelled. In certain older conventional systems, chip bins at atmospheric pressure have been used, in which the chips are pre-heated with steam in order to expel the air. Very large volumes of expelled air are obtained from these systems, and this air is contaminated with turpentine, methanol and other explosive gases. If steam is used that has been obtained from the release of pressure from black liquor, this steam contains also large quantities of sulphides known as “TRS gases” (where “TRS” is an abbreviation for “total reduced sulphur”). These sulphides are very foul-smelling. These TRS gases contain, among other compounds, hydrogen sulphide (H2S), methyl mercaptan (CH3SH), dimethyl sulphide (CH3SCH3), dimethyl disulphide (CH3SSCH3), and other gases that are strongly foul-smelling or explosive. Hydrogen sulphide and methyl mercaptan arise to a major degree from the vaporisation of black liquor, and the boiling points of these are −60° C. and +6° C., respectively. This means that it is difficult to separate them from the gases by condensation.
The gases that do not lend themselves to easy removal by condensation are known as “NCGs” (where “NCG” is an abbreviation for “non-condensable gas”).
Pure steam is often used for heating in the chip bin in order to minimise the release of TRS gases, and the black liquor steam is used first in a pressurised steam pre-treatment vessel that is located after the chip bin. Even if the black liquor steam is used solely in a subsequent pressurised steam pre-treatment vessel, these TRS gases can leak up to the chip bin, for example, during interruptions in operation. The use of pure steam for the steam pre-treatment, however, is expensive since the amount of steam available for the production of electricity at the pulp mill is in this case reduced.
Steam is driven through the complete bed of chips in certain steam pre-treatment systems, and this means that large volumes of dilute weak gases are obtained that must be managed in what are known as “weak gas systems”. These steam pre-treatment systems are often known as “blow-through” systems, where the temperature in the uppermost surface of the bed of chips, or in the gas phase above the chips, or at both of these locations, is considerably higher than the ambient temperature, normally around 60-100° C. One major disadvantage of these systems is that a major fraction of the steam energy that is supplied is expelled with the expelled gases. These gases are condensed in weak gas systems with the result that large amounts of low-grade warm water are obtained, which often is passed to the drainage system, leading to large losses of energy.
The prior art technology has identified the problem as being that of desiring to minimise the leakage of harmful or toxic gases that arise during the steam pre-treatment using hot steam. There is normally a transfer of weak gases from the chip bin to a destruction system, and a further release transfer of gases from the steam pre-treatment vessel, the latter gases being often regarded as strong gases. It is normally attempted to maintain the concentration of the weak gases to a value well under 4% by volume, and that of the strong gases to well above 40% by volume.
In known chip bins in which steam is blown into the bed of chips, large amounts of gases are generated, and either pure steam or special systems that can deal with these gases are required. Expelled gases may easily acquire a very explosive composition. There is no risk of explosion as long as the concentration of the gases lies under approximately 4% by volume or well over 40% by volume. For this reason, either weak gas systems that maintain a concentration of under 4% by volume, typically 1-2% by volume, or strong gas systems that maintain a concentration of well over 40% by volume are used. Thus, it is ensured in weak gas systems that the concentration is held well under 4% by volume, and this entails the transport of large amounts of air. As soon as the amount of gases is to increase, a corresponding increase in the amount of air must be carried out in order to maintain the concentration under the critical level.
If, for example, 1 kg/min of NCGs is created by steam pre-treatment in a chip bin, the amount of air must lie at around 50 kg/min in order to maintain a concentration of approximately 2% by volume. If the amount of NCGs were to increase to 2 or 3 kg/min, as may occur in the event of certain disturbances in the process, the amount of air must temporarily be increased to 100 or 150 kg/min, respectively. This results in the systems normally being dimensioned such that they can deal with the normal flow, while excess gases that arise during interruptions in operation are expelled directly to the atmosphere through vent pipes.
A further solution for minimising the volumes of weak gases is to control the flow of chips through the chip bin such that a stable plug flow through the chip bin is established, and where the addition of steam to the chip bin takes place in a controlled manner such that only the chips in the lower part of the bin are heated to 100° C., while the temperature in the gas phase above the chips level that is established in the steam pre-treatment bin essentially corresponds to the ambient temperature. This technique is known as “cold-top” control and it is used in chip bins that are marketed by Metso Paper under the name of DUALSTEAM™ bins, and that are used in impregnation vessels that are marketed under the name of IMPBIN™. The major advantage of these systems is that they give heating in an efficient manner, in which all of the supplied heat is absorbed into the process. This is in contrast to heating in which the steam is allowed to blow away through the upper surface of the bed of chips and where the vented steam must be condensed, giving large losses of energy. A further advantage of “cold-top” control is that a further location is not established in the process at which the loss of turpentines from the chips can take place, and for this reason essentially all turpentine accompanies the black liquor that is withdrawn from the digestion process. The pressure of this black liquor can then be released in a conventional manner in a flash tank or in the evaporation process.
A number of very expensive solutions have been developed in order to reduce the explosiveness and toxicity of the gases. WO 96/32531 and U.S. Pat. No. 6,176,971, for example, reveal different systems in which digester liquor drawn off from the digester generates pure steam from normal water. The TRS content of the weak gases is reduced by using totally pure steam for the steam pre-treatment of the chips, since the steam used is totally free of any TRS content. These systems, however, inevitably give rise to loss of energy and more expensive process equipment.
SE 528116 (WO2007064296) reveals an embodiment for the handling of the weak gases that are expelled from a chip bin with cold-top control. Air is in this case added to the weak gas system at an amount that is proportional to the degree of blow-through, such that the weak gases remain at all times on the dilute side of the region of concentration at which they become explosive. A gas washing operation is here included in the weak gas system.
The steam treatment of chips in the prior art technology has had the principal aim of expelling air from the chips, and the possibility of using cooling fluids directly in the steam treatment has for this reason not been considered. The cooling technique has been used exclusively in the subsequent weak gas system, which is independent of the steam pre-treatment vessel, where the gases have been cooled or condensed. It has, however, proved to be the case that the use of cooling fluids during the steam treatment is very efficient, and that relatively small amounts of cooling fluid are required in order to eliminate problems with odour. Since disturbances in the system occur sporadically, it is simple to avoid the dilution effects in the weak gas systems described above, with the use of direct cooling.
A first object of the invention is to make the steam pre-treatment process safer such that the risk of blow-through of the chips is reduced to a minimum, and this in turn ensures that the release of foul-smelling gases to the surroundings can be kept to a minimum.
A second object is to ensure that the layer of condensate in the bed of chips is kept at a safe level in the volume of chips, and that it does not reach the upper surface of the volume of chips where this condensate can be converted to gas.
A third object is that the safety system should preferably be used during what is known as “cold-top” control during steam pre-treatment of the chips, where the chips are heated such that a temperature gradient is formed in the volume of chips, where the chips at the top of the chip bin have the ambient temperature, typically around 0-50° C., preferably 20-40° C., and a gradually higher temperature is established down towards the bottom of the chip bin, with an advantageous temperature of approximately 90-110° C. established at the bottom of the chip bin. This system has the result that the volumes of gas that are expelled from the chips in the chip bin are very low, and the load on the weak gas system will be minimal during continuous equilibrium operation. One property of the system, however, is that expelled gases tend to condense in a condensation layer within the volume of chips. The risk of steam blow-through can be significantly reduced, however, by the use of a simple cooling process for the chips.
A fourth object is to minimise the effects of a blow-through, should such occur, by replacing the cooling surface of the chips by an amount of cold fluid, on which the amount of foul-smelling gases released can be reduced to a minimum, while the total duration of the release can be significantly reduced.
The objects described above are achieved with an arrangement according to the present invention.